1. Field of the Invention
This invention relates to a wavelength tunable laser in which the oscillation wavelength is variable and an optical tomography system which obtains a tomographic image of the object to be measured by the use of the wavelength tunable laser.
2. Description of the Related Art
As a wavelength tunable laser whose oscillation wavelength is tunable, there has been known, for instance, an external resonator type laser shown in FIG. 12. (See U.S. Patent Application Publication No. 20050035295) In the laser shown in FIG. 12, light emitted from a low reflection face of a laser medium 111 is made parallel by a collimator lens 112 and then caused to enter a diffractive optics 113. The diffractive light which is spatially dispersed by the wavelength by the diffractive optics 113 is caused to enter a polygon mirror 125 by way of a pair of lenses 124a and 124b. Out of diffractive light which undergoes the wavelength dispersion by the diffractive optics 113, only components of a specific wavelength and those close thereto which are perpendicular to the reflecting surface of the polygon mirror 125 form return light and return to the semiconductor laser medium 111. The semiconductor laser medium 111 makes standing waves and emits light of the specific wavelength (to be referred to as “oscillating wavelength” hereinbelow. By rotating the polygon mirror 125, the wavelength of the return light can be continuously changed, whereby the oscillating wavelength can be swept. In the system shown in FIG. 12, the wavelength changes with time in proportion to sin θ (θ is an inclination angle to the optical axis). Further, in U.S. Patent Application Publication No. 20050035295, there is disclosed a wavelength tunable laser, where a lens 134 and a rotary disk 135 are substituted for the lenses 124a and 124b and the polygon mirror 125 shown in FIG. 12 as shown in FIG. 13. In the laser, only light of a particular wavelength returns to the semiconductor laser medium 111 by virtue of the slit-like mirrors 135a which are disposed on the surface of the rotary disk 135 to diametrically linearly extend. By rotating the rotary disk 135, the wavelength of the light returning to the semiconductor laser medium 111 can be continuously changed, whereby the oscillating wavelength can be swept. In the system shown in FIG. 13, the wavelength changes with time in proportion to tan θ (θ is rotating angle of the rotary disk 135), and substantially linearly changes.
In “Amplified, frequency swept lasers for frequency domain reflectometry and OCT imaging: design and scaling principles”, R. Huber et al., OPTICS EXPRESS, Vol. 13, No. 9, pp. 3513-3528, 2005 there is further disclosed, as another system, a laser in which a tunable Fabry-Perot filter 143 is employed to select the oscillating wavelength in a fiber-ring resonator where an optical fiber 142 is connected to a semiconductor optical amplifier 141 at its opposite ends. In this system, the direction in which light passes is limited by a pair of isolators 144 and 145, and a laser beam is externally output from an optical coupler 146 provided in a part of the fiber-ring. In the system shown in FIG. 14, the wavelength changes with time is like a sine-wave by virtue of the characteristics of the tunable Fabry-Perot filter.
As an important application of the laser where the wavelength can be swept, there has been known an optical tomography system employing an SS-OCT (swept-source OCT) measurement. In the optical tomography system, coherence light emitted from a light source is divided into measuring light and reference light and the reflected light when the measuring light is projected onto the object and the reference light is multiplexed, whereby a tomographic image is obtained on the basis of the intensity of the interference light. In the SS-OCT optical tomography system, the interference light is detected while the frequency of the light emitted from the light source is changed with time, and the reflected light and the reference light are caused to interfere with each other while the frequency of the laser beam emitted from the light source is changed with time by the use of a Michelson interferometer. Then reflection intensity in a predetermined position in the direction of depth of the object is detected on the basis of the interferogram in the region of an optical frequency, and a tomographic image is generated by the use of the reflection intensity.
On the other hand, such an optical tomography system is applied to an endoscope and is employed in determinate diagnosis in a living body, a diagnosis of the depth of the cancer such as distinguishment between mucous membrane cancer (m cancer) and sub-mucous cancer (sm cancer), or the like. Procedure of cancer diagnosis under an endoscope will be briefly described, hereinbelow. First, a diseased part is found on the basis of a normal observing image, and the diseased part is distinguished whether it is a cancer. This primary diagnosis is based on experience of the doctor and whether the part is a cancer is determined by way of a pathology examination on the tissue of the part to be diagnosed as a cancer. Accordingly, determinate diagnosis during the examination through the endoscope is difficult under the present situation. When a human is diagnosed as having cancer, the depth of the cancer is examined again through an endoscope in order to determine a course of treatment. Generally, cancer is generated from the surface of a mucous membrane and infiltrates into a direction of depth while laterally expanding in response to progression thereof.
As shown in FIG. 15, the stomach wall comprises 5 layers, a mucous membrane layer (m layer), a mucous myotome (MM), a sub-mucous layer (sm layer), a muscular layer and a serous membrane layer. Cancer which stays a mucous membrane layer is called m cancer, while cancer which filtrates to the sub-mucous layer is called sm cancer. m cancer and sm cancer are different in the method of treatment. In the case of sm cancer since the lymphoid systems and/or the blood vessel systems exist in a sub-mucous layer and a probability of metastasis cannot be denied, a surgical operation is applied. On the other hand, in the case of m cancer, since a probability of metastasis is null, the cancer is extracted under an endoscope. Accordingly, it is important to distinguish the m cancer and the sm cancer. Specifically it is important for the image to be able to be evaluated whether the mucous myotome (MM layer) holds a layer structure or has been broken. At present, application of ultrasonic sound is investigated having in view on the diagnosis of the depth of the cancer. However, since the axial direction resolution is about 100 μm or so and imaging of the MM layer is insufficient, there is a demand for putting into practice a method of the optical tomography where the axial direction resolution at a depth of 1 mm is not larger than 30 μm.
In an optical tomography system using SS-OCT (swept source OCT) measurement, the axial direction resolution at a depth of 0 mm is governed by the sweeping wavelength band width and the central wavelength of the wavelength sweeping of the measuring light. Widening the measuring light sweeping wavelength band width has been progressed. As a result, an axial direction resolution of 10 μm or so can be obtained at a depth of 0 mm at the present.
However, in the wavelength tunable lasers shown in FIGS. 12 to 14, the wavelength change with time depicts a sin θ curve (θ representing the inclination angle from the optical axis) or a curve like a sine curve. In the above described optical tomography systems using SS-OCT (swept source OCT) measurement, there is often carried out on obtained data frequency analysis by the use of Fourier-transform and what is used as a variable when frequency analysis is carried out on obtained data by the use of Fourier-transform is not a wavelength but a frequency. There has been known that the axial direction resolution deteriorates as the measuring depth increases when data discretely distribute with respect to the variable in the analysis.
FIG. 16 shows a result of simulation where linearity of the frequency change with time of the measuring light and state of deterioration of the signal at a depth of 1 mm are calculated in such an optical tomography system. The linearity of the frequency change with time (sometimes referred to as “linearity of the frequency change”, hereinbelow) is in terms of % while the difference between 0° and the real measured value of 30° when the polygon mirror 125 is rotated by 30° in the system shown in FIG. 12 is taken as a denominator and a maximum value of the part where the frequency change with time deviates from an ideal linear line when the polygon mirror 125 is rotated by 30° in the system shown in FIG. 12 is taken as a numerator. Further, when the wavelength change with time is linear, linearity of the frequency change is 4.7%.
As can be seen from FIG. 16, when the wavelength change with time of the measuring light is linear, that is, when the linearity of the frequency change with time of the measuring light is 4.7%, the deterioration of the signal at a depth of 1 mm and it is difficult to obtain an optical tomographic image by the use of such measuring light.